1. Technical Field
The present invention relates to a storage device for storing data used in computers and diversity of other electronic devices.
2. Related Art
Memory cards like compact flashes (registered trademark) and USB flash drives have widely been used as external storage devices of computers. Flash memories as rewritable nonvolatile ROMs are included in these external storage devices. There are two types of flash memories, SLC (single level cell) flash memories and MLC (multi level cell) flash memories (see, for example, JP-A-2002-8380).
The widely used SLC flash memories have the capacity of storing 1-bit information per unit cell (these flash memories are also referred to as binary memories). The MLC flash memories have the capacity of storing at least 2-bit information per unit cell (these flash memories are also referred to as multivalued memories). The MLC flash memory takes four different charging states, the fully charged state, the ⅔ charged state, the ⅓ charged state, and the fully discharged state, to store 2-bit information per unit cell.
The general features of the MLC flash memory are the larger storage capacity but the lower processing speed and the lower rewritable capability than the SLC flash memory. The general features of the SLC flash memory are the higher processing speed and the higher rewritable capability but the smaller storage capacity than the MLC flash memory.